Conventional wisdom generally around 680 is the breakpoint to start looking at conventional.
FHA rates are published, and you can mostly calculate the conventional ones from:
https://www.fanniemae.com/content/pricing/llpa-matrix.pdfNamely 5% down on a confirming primary SFH will cost you 1.25% more in APR at 640 instead of 680.
Very handy document to find out the breakpoints, absolutely used it to throw 25% for my down payment on a 15 year mortgage to optimize rates at the time.